The present invention relates to techniques for analyzing ions by time-of flight mass spectrometry, and more particularly to techniques for analyzing ions by mass spectrometry that involves calibration of a mass spectrometer.
Mass spectrometry is a significant tool useful for analyzing ions. The knowledge of the masses and relative abundance of the various fragments produced after a ionized compound breaks down helps an investigator in determining the chemical structure of an unknown compound. If the compound has been analyzed with mass spectrometry, searching a mass spectral library may help to identify the compound.
In traditional mass spectrometry, the ions go through an electrostatic, magnetic or electromagnetic (quadrupole for instance) filter that only lets through ions of a given mass. The ions are then detected. The filter is tuned to different masses and the experiment repeated until all the masses of interest have been measured. Sensitivity often is not as good as desired because at a given time, except those ions of the mass allowed through the filter, all others are discarded.
In time of flight mass spectroscopy (TOF-MS), a packet of ions is launched by an electrostatic pulse towards a detector a distance away. Ions having the same initial kinetic energy but different masses will separate when allowed to drift along a field-free region. The ions are given either equal momentum or equal energy, and they separate in flight according to their masses, the heavy ions arriving behind the light ions. By measuring the flight times, one can know the masses of the various ions in the packet. Because each packet contains only a few ions, the experiment is repeated many times and the measurements are summed in order to increase sensitivity. After a few hundred to many thousand cycles, which may take only a fraction of a second, the quality of the measurement is sufficient to identify the compound. The ions of all masses are analyzed in parallel instead of one mass at a time. Patents of general interest on TOF-MS include, for example, U.S. Pat. No. 5,847,385 (Dresch), U.S. Pat. No. 5,852,295 (Da Silveira et al.), and U.S. Pat. No. 5,898,174 (Franzen), which are incorporated by reference in their entireties herein.
A graph representing the mass spectrum results of TOF-MS showing ion abundance as a function of time of flight contains numerous peaks. To obtain the correct masses from a mass spectrum, one needs to convert the peaks in the mass spectrum to the corresponding masses. Such a conversion process, generally, involves a calibration step. Given a particular set of equipment, TOF-MS calibration would establish a time to mass conversion formula that a user will be able to obtain an ion abundance versus mass relationship from a mass spectrum, instead of an ion abundance versus time relationship. Typically, calibration of a mass spectrometer is done by the injection of a sample of known composition, for example, HCB (hexachlorobenzene) into the mass spectrometer. As an alternative, one may rely on the assumption that the residual signal in the absence of any analyte or unknown sample is from mostly air and water. In such cases one can recognize at least two peaks in the mass spectrum, then solve for the calibration equation. However, in many industrial circumstances, for example, in the production of semiconductor material, even trace amounts of contamination can be problematic and injecting a calibration compound increases the risk of contamination. Thus, there is a need for a calibration method without the introduction of calibration samples of a known chemical nature into the mass spectrometer.
This invention provides techniques for analyzing ions by determining the time of flight of the ions from a source before detection at a detector. In this technique, the calibration of a time-of-flight mass spectrometer (TOF-MS) can be done without the introduction of calibration compounds of known chemical nature into the mass spectrometer. In one aspect, the present invention provides a method for internal calibration in a TOF-MS. To analyze ions by determining the time of flight of the ions from a source before detection at a detector in a TOF-MS, the present invention uses a calibration method that includes launching a packet of ions from a source to travel a distance to a detector, detecting the time of arrival of the ions at the detector to obtain a time-of-flight mass spectrum thereof, and selecting data from the mass spectrum corresponding to a plurality of ions of consecutive masses and using these selected data to determine the relationship between time of flight data and the masses of the ions of consecutive masses for calibration of the relation between the time of flight data and masses of ions in the mass spectrometer. In another aspect, the present invention provides a TOF-MS that can calibrate internally without the need for injecting a calibration compound of a known chemical nature. In such a TOF-MS, a processor determines the calibration by selecting data corresponding to a plurality of ions of consecutive masses and using these selected data to determine the relations of time of flight data and masses of ions.
The techniques of the present can be advantageously used for significantly increasing the reliability of TOF-MS. Applying the present technique of internal calibration, i.e., without the injection of a calibration compound of known chemical nature into the mass spectrometer, the risk of contamination is significantly reduced. Further, the present method can be used to compute the calibration quickly, since the present method requires no a priori knowledge on the instrument settings or the chemical being analyzed.